1. Field of the Invention
The present invention generally relates to a four-cycle engine, and more particularly relates to a four-cycle engine having an overhead camshaft drive.
2. Description of Related Art
Four-cycle engines in outboard motors have become more common in order to control emissions more precisely. Such four-cycle engines typically comprise a crankshaft that drives a submerged marine propulsion device through suitable shaft couplings. Modern four-cycle engines typically employ an overhead camshaft drive mechanism. The camshaft drive mechanism drives at least one camshaft which actuates an intake and/or exhaust valve(s). Normally, at least one flexible transmitter e.g., a drive belt or chain that is disposed atop the engine, drives the camshaft(s).
The four-cycle engine also can have a variable valve timing (VVT) system to obtain high charging efficiency in a relatively high engine speed range and low fuel consumption and superior exhaust characteristics in a relatively low engine speed range. The VVT system can change valve timings of either the intake or exhaust valve(s) in response to the engine speeds. The VVT system can be operated hydraulically and control valve unit can control the hydraulic operation of the VVT system. Normally, the VVT system is disposed at the camshaft associated with the intake or exhaust valve(s) that needs the valve timing change. The control valve unit, more specifically, controls flow of working fluid (e.g., hydraulic fluid) in the VVT system to change an angular position of the camshaft. Such a VVT system is disclosed in, for example, Japanese Laid Open Patent Application P2001-35 5466A.
Some of the control valve units in conventional arrangements are disposed on a bearing cap that rotatably supports the camshaft on a cylinder head of the engine. Due to length of the control valve unit, the bearing cap that has the control valve unit can cause the engine to be larger.